Ultra high frequency resonant circuit



NOV. 14, 1950 c, sMlTH 2,530,089

ULTRA HIGH FREQUENCY RESONANT CIRCUIT Filed June 28. 1946 INVENTORATTORNEY Patented Nov. 14, 1950 ULTRA HIGH FREQUENCY RESONANT CIRCUITLester C. Smith, Westmont, N. .l., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application June 28, 1946, Serial No.679,973

This invention relates to resonant circuits, and particularly to aresonant circuit of the concentric line type.

Among other objects of the invention are:

To provide a resonant circuit for use above 300 megacycles which has afrequency tuning range greater than two and somewhat less than 3;

To provide a concentric line resonator which eliminates the need forsliding contacts to engage the inner and outer conductors and which hasa wider tuning frequency range than known concentric line resonantcircuits which do not have sliding contacts.

' To provide a concentric line type of resonator which is more compactthan a cavity resonator for the same frequency of operation and whichrequires a relatively smaller mechanical motion to cover the entiretuning range than known types of concentric line resonators.

A more detailed description of the invention follows in conjunction witha drawing wherein:

Fig. 1 illustrates a tunable resonator circuit in accordance with oneembodiment of the invention, and

7 Figs. 2a, 2b and 2c illustrate the resonant circuit of Fig. 1 forthree different conditions of tuning.

Referring to Fig. 1, there is shown a concentric line type of resonantcircuit in accordance with theinvention, having an outer conductor 8 anda concentric inner conductor composed of a member ll] terminatingwithout conductors, as shown, and a cup-shaped member I2. The cup-shapedmember [2 comprises a hollow inner conductor H which is slightly largerthan the diameter of the member l8, and a surrounding outer hollowconductor I3 which is slightly smaller in diameter than the hollowconductor 8. If conductor 8 and conductor ID are circular incross-section then it is preferred that conductors H and i3 of cup [2also be circular in cross section. If, on the other hand, conductors 8and H] are rectangular in cross-section, it is preferred that conductorsl l and I3 also be rectangular in crosssection.

Rod I is supported at its center and maintained in spaced relation withrespect to the outer conductor 8 by means of an insulating ring ct-spider. =1 fixedlypositioned between the con- 4 Claims. (Cl. 178-44)ductors 8 and It]. The conductor f l' is sum-- ciently large to permitmovement of this conductor over the right-hand end of conductor I0without making contact with this rod. The capacity between the conductorI0 and the conductor H is designated by C2. The hollow conductor [3 ofthe cup 12 is similarly spaced from. the surrounding outer conductor 8and forms therewith a capacity 01. The cup I2 is movable along the axisof the resonant circuit in the direc-- tion of the arrows by means of aninsulating rod 9 which is secured to the cup and which is slidablewithin an end plate [4 attached to the surrounding outer conductor 8. Itshould be noted that as the conductor [2 is moved toward or away fromthe right-hand end of the conductor III, the capacity C2 will vary froma very low value to a relatively high value.

In practice, the length of the cup [2 which is designated Ll isapproximately one-half the length of the rod II] which is designated L2.The length L2 of the rod I0 is one-half wave length at the highestfrequency of operation, while the length LI of cup I2 is less thanone-fourth of a wave length at the highest frequency of operation.

An inspection of Fig. 1 will show that when the cup [2 is arranged inthe position shown, the inductance of conductor [0 will be in serieswith the capacitor C2, in series with the inductances of hollowconductors l l and I2, and in series with the capacitor Cl. Capacitor CIis a relatively large capacity and has a value appreciably larger thancapacitor C2.

Fig. 2a illustrates the voltage distribution curve of the resonantcircuit when the cup I2 is pulled away from the right-hand end of rodIt! so as to have no effect on the resonant circuit. In this condition,the voltage curve of the resonant circuit is indicated by V whichrepresents the distribution of the voltage along the rod 10. In thisposition, the frequency of the resonant circuit is the highest.Capacitors Cl and C2 do not enter into consideration in the particularposition of Fig. 2a because of the large spacing between conductor l0and cup l2.

Fig. 2b illustrates the condition where'the cup 12 is suficiently closeto rod I0 so that the capacity between the rod l0 and cup l2 tunes outthe inductance of the conductors of cup l2. In this.

condition, the system is so tuned that the frequency of operationcorresponds to rod being one-fourth of a wave length. This frequency isan intermediate frequency for the tuning range of the resonant circuit.

Fig. 2c shows the condition of tuning which is opposite to that of Fig.2a; namely, where the tuned circuit is resonant to the lowest frequencyof operation in the tuning range. An inspection of the voltage curvedistribution V of Fig. 2c will show that the entire length of LI plus L2corresponds to one-fourth of a wave length at the operating frequency.

As an illustration only, assuming that the resonant circuit of theinvention is designed to operate over a frequency range of 300megacycles to 850 megacycles, L2 will be slightly larger than one-sixthof a meter long, while Ll will be slightly less than one-twelfth ofameter, both LI and L2 together having a total length of onefourth of ameter. It will thus be seen that L2 is approximately twice as long asLI.

In practice, it is necessary to move the cup I2 only a very shortdistance in order to overlap the right-hand end of rod for obtaining thelowest frequency of operation in the tuning range. This is shown in Fig.20. Any further increase in capacityof C2.causedby further movement ofthe cup 12 over the rod 10 will cause further shortening of theeffective length of the resonant circuit'and hence, increase infrequency. In using the resonator, of-the invention, itis preferred thatthe point of -minimum frequency not be passed by moving cup 12 too farto the left over theadjacent end of rod 10-. The preferredmodusoperandi-is-to have the tuning range extend from the conditionshown in Fig. 2a where L2 equalsone-half wave length at the highestfrequencytothe condition shown in Fig. 2c where L! plus L2 equalsone-fourth of a wavelength at the lowest frequency. In the variationbetween 'thehighest frequency to the lowest frequency in the tuningrange, the operation issmooth and \continuous.

The resonant circuitof the present-invention eliminates the needfo'rsliding mechanical contacts betweeninner and outer conductors and:requires only a. small mechanical motion to cover the entire tuning.range of frequencies. By way of illustration, the motion of cup 12needbe only one-half inch to cover a range from 300 megacy-* cles to 850'megacycles with suitable dimensions of the. conductors and.suitablespacing' of these conductors to provide condensers C2 and Cl,

whereas the conventional type of resonant circuit may needa movement of.five inches or more to cover a considerably-narrower band offrequencies. It has: further been found that the system of the inventionis much morev compact than a cavity resonator designed to operate at anequivalent frequency of operation.

A feature of the present invention lies in the fact that the resonantcircuit decreases in frequency with decreasing overall effective lengthwhich is a condition opposite to concentric line tuned circuitswherein'aa decrease in frequency requires an increase in effectivelengthof the tuned circuit.

The resonant circuitof the invention is not limited to any particularrangeof frequencies and may be designed to operate anywhere in the rangeupto10,000megacycles. The tuning range of the resonant circuit of theinvention is about'2.8 to 1 which provides a frequencycoverageconsiderably "greater than -khown=--types of concentric assume 4 linesresonators which do not use sliding contacts. Thus a resonator inaccordance with the invention can be constructed over a range of 300megacycles to 850 megacycles or a range of 1,000 megacycles to 2,800megacycles.

The load may be coupled to the resonator of the invention by connectingthe grid and anode of a vacuum tube to the left-hand end of the outerconductor 8 and the rod H! where the resonant impedance is large, or byinserting a loop between the outer conductor 8 and the rod ID at anintermediate position where the-current at resonance isrelatively'large'.

What is claimed is:

1. A tunable resonant circuit comprising a coaxial line having an outerconductor and an inner conductor terminated therewithin, and a tuningstructure axially movable with respect to said line and comprising atubular outer conductor spaced from and capacitively coupled to saidfirst outer conductor and a coextensive tubular inner conductor withinsaid outer conductors, a direct connection between said tubularconductors at one end, said tubular inner conductor having a hollow endportion at the end opposite said direct connection and facing. thetermination of said first'inner conductor, the cross section of saidtubular inner conductor being sufficiently different from said-firstinner conductor'topermit overlapping without contact therebetween with aconsequent capacitive couplin therebetween.

2. A-tunable resonant circuit comprising a coaxial line having an outerconductor and an in-' ner conductor terminated therewithin, and a tuningstructure axially movablewith respect to saidline and comprising atubular outer conductor insulatingly spaced from and within said firstouter conductor and capacitively coupled thereto and a substantiallycoextensive tubularzinner-conduc tor within said first-outer conductorand having a physical length approximately one-half that of said firstinner'cond-uctor, said tuning; structure having: a:member shortcircuiting said tubularconductors'at one end, said tubular i1iner-=con-'ductor havingea' hollow end portion facing. the terminaticnof and of adifferent cross-section than said first inner conductor to capacitively.couple. said-sinner conductors byxvirtue of l. the spacing therebetween,the capa'citybetween said: outer conductors being greater than thecapacity between said inner conductors over the entire tuning range;

3. The tunable resonant circuit claimedin claim 2, said hollow endportion havin'g an inner diameter-greater than the outer diameter oisaid first inner conductor and said movable tuning structure beingaxially movable to brlngsaid hollow portion around the terminatingportion ofsaid first'inner conductor and spaced therefrom;-

4.- A tunable resonant circuit comprising: a- 00-- axial line having:an' outer conductor and an in-' ner conductor-terminated therewithin,.said linehaving an: electrical length substantially equal toa halfwavelength at the highestoperating fre quency, and a tuning -structureaxially movablewithin said outer conductor and comprising a conductiveend plate, a tubular outercondu'ctor physically spaced from and adjacentto said-first outer conductor to be capacitively coupled thereto andconductively connected to and extending from said end plate and atubular inner-con ductorwithin said tubular outer conductorandconductively. connected to and' extending from said end plate, one ofsaid inner conductors hav-' ing a hollow terminating portiorr withinvvl'iichs 2,530,089 5 6 the terminating portion of the other said innerREFERENCES CITED conductor may be received and in which when so receivedthe one end portion is closely spaced from the other end portionreceived by it to The following references are of record in the file ofthis patent:

capacitively couple together said inner conduc- 5 UNITED STATES PATENTStors, said tubular inner conductor having a length Number Name Dateslightly less than a quarter wavelength at the 2,285,662 Hutcheson June9, 1942 highest operating frequency, both inner conduC- 2,408,895 TurnerOct. 8, 1946 tors together having a total length of one-quar- 2,411,299Sloan Nov. 19, 1946 ter wavelength at the lowest operating frequency, 10the capacity between said outer conductors being greater than thecapacity between said inner conductors over the entire tuning range.

LESTER 0. SMITH.

